1. Field of the Invention
This invention relates to neutron generating systems and more particularly pertains to sealed-tube neutron generators especially adapted to traverse the narrow confines of a well or borehole, although useful in a variety of other applications.
2. The Related Art
The use of a generator of high energy neutrons has been known for a long time for neutron-gamma ray or neutron-neutron logging in oil well logging tools. Accordingly, for illustrative purposes, the invention is described in more complete detail in connection with a sealed-tube neutron generator suitable for use in a well logging tool.
Sealed-tube neutron generators usually have four major features:                (i) a gas source to supply the reacting elements, such as deuterium and tritium;        (ii) an ion source that strips electrons from the gas molecules, thus generating a plasma of electrons and positively charged ions;        iii) a target impregnated with deuterium and/or tritium; and        (iv) an accelerating gap which propels the ions from the plasma to the target with such energy that the bombarding ions collide and fuse with the deuterium or tritium nuclei of the target to generate and emit neutrons therefrom.        
Ordinarily, a plasma of positively charged ions and electrons is produced by energetic collisions of electrons and uncharged gas molecules within the ion source. Two types of ion sources are typically used in neutron generators for well logging tools: a cold cathode (a.k.a. Penning) ion source and a hot (a.k.a. thermionic) cathode ion source. These ion sources employ anode and cathode electrodes of different potential that contribute to plasma production by accelerating electrons to energy higher than the ionization potential of the gas. Collisions of those energetic electrons with gas molecules produce additional electrons and ions.
Penning ion sources increase collision efficiency by lengthening the distance that the electrons travel within the ion source before they are neutralized by striking a positive electrode. The electron path length is increased by establishing a magnetic field which is perpendicular to the electric field within the ion source. The combined magnetic and electrical fields cause the electrons to describe a helical path within the ion source which substantially increases the distance traveled by the electrons within the ion source and thus enhances the collision probability and therefore the ionization and dissociation efficiency of the device. Examples of neutron generators including Penning ion sources used in logging tools are described e.g. in U.S. Pat. No. 3,546,512 or U.S. Pat. No. 3,756,682 both assigned to Schlumberger Technology Corporation.
Hot cathode ion sources comprise a dispenser cathode realized from a material that emits electrons when heated. A grid electrode extracts electrons from the cathode which, in turn, ionize the gas, generating ions. An extracting or focusing electrode extracts ions and focuses such ions so as to form an ion beam. An example of a neutron generator including a hot cathode ion source used in a logging tool is described e.g. in U.S. Pat. No. 5,293,410, assigned to Schlumberger Technology Corporation.
In these systems, the target floats at a negative high voltage potential, typically on the order of −70kV to −160kV (or less), with the ion source electrodes operating around ground potential, in order to provide the necessary electric field gradient to accelerate ions toward the target with enough energy that the bombarding ions generate and emit neutrons therefrom. Typically, on the order of 10 watts of power are dissipated in the target and the target is surrounded by high voltage insulation. Because of poor thermal conduction to the exterior (due to the fact that electrical insulators are generally poor thermal conductors), the temperature of the target can increase significantly compared to ambient temperature. At high ambient temperature, the target can overheat, leading to failure (loss of neutron output) of the neutron generator.